The invention relates to ceramic installations and more particularly to ceramic reflectors for use in nuclear reactors.
Ceramic installations are utilized in gas-cooled, high temperature reactors as side, bottom and roof reflectors intended to reduce losses caused by the migrating of neutrons. At least part of the neutrons moving to the outside are to be reflected back into the fission zone of the reactor, the so-called reactor core. The neutron flux increased at the edge of the fission zone by the reflector effect raises the output produced by unit mass of the fissionable material, leading to the improved utilization and more economical consumption of the nuclear fuel.
Highly purified graphite is used in gas-cooled, high temperature nuclear reactors as the material for ceramic installations. It is relatively inexpensive, has adequate strength and may be processed mechanically. It is refractory and has good thermal conductivity. A disadvantage is the change in its crystalline structure caused by neutron and gamma radiation and expressed by changes in mechanical strength and in volume.
Under the effect of temperature and high neutron fluxes, graphite expands at first negatively, but then positively beginning at a point of reversal, with increasing fluxes, which go beyond the original dimensions of the graphite body. The process is displaced with rising temperatures toward lower fluxes.
The differences in expansion within the structural part--in keeping with the flux distribution, at the onset of the irradiation the block layers close to the surface of the front facing the core tend to become shorter to a relatively greater extent due to shrinkage than the deeper layers--are the cause of the generation of residual stresses. To reduce these residual stresses, it is necessary to provide relief by expansion equalization. This may be obtained advantageously by slit surface structures which are equivalent to a dimensional reduction of parts of the graphite block.
In more recent developments of gas-cooled, high temperature nuclear reactors, in particular those of lower capacity (approximately 100 MWel) and with correspondingly smaller core diameters, in place of absorber rods inserted directly into the pile of spherical fuel elements, small absorber elements in the spherical form are provided for the shutdown of the reactor and are introduced into corresponding cavities of the reflectors.
In a manner similar to the AVR nuclear power plant in Julich, in the core of which nose-like projections with vertical cavities, the so-called nose stones, are provided to receive the control and shutdown rods, the newer gas-cooled high temperature nuclear reactors presently in the planning stage are equipped with nose stones of this type, but here they are intended for shutdown rods only. The nose stones are ashlar-shaped graphite blocks extending radially from the side reflector, to which they are physically joined, over the entire height of the reactor core into which they are projecting.
In view of the above-mentioned volume variations and the residual stress state caused by them in the irradiated graphite blocks, the surfaces facing the core of the latter are provided with vertical and horizontal surface slits, representing a resolution of the original large surface into small individual rills. To control stresses in the nose stones, the cavities provided for the introduction of the absorber elements are connected with the core by means of gaplike, vertically disposed continuous openings. These openings reduce the residual stresses in the nose stones to tolerable levels.
However, the afore-mentioned expansions lead in the course of the operation to a widening of the openings to such an extent that the separation of the absorber material and the fuel elements is no longer assured and the absorber elements are able to exit from the cavities and fuel elements can enter them.
Based on this state of the art, it is the object of the invention to provide measures for the design of ceramic installations which are simply and cost effectively realized and which prevent in a highly reliable manner the aforementioned deficiencies, in particular the passage of absorber and/or fuel elements through the continuous opening.